The present invention relates generally to multi-color imaging technology and, more particularly, to techniques for recalibrating a multi-color imaging system.
A multi-color imaging system typically is configured to form a reproduction of a multi-color image by application of a plurality of different colorants to a substrate. The multi-color image is defined by a set of color separations containing color values. Each color value represents an intensity for one of the separated colors at a particular pixel in the original image. Thus, each pixel is defined by a set of color values, each representing the intensity of a different separated color in the pixel.
The imaging system uses the separated color values to control the amount of each colorant applied to each pixel on the substrate. In an inkjet printing system, for example, the color values are used to control the amount of ink deposited on a sheet of paper. In a thermal dye transfer printing system, the color values may be used to control a thermal head or laser to effect the transfer of an amount of dye from a donor to a receptor. In either case, the imaging system applies to the substrate a combination of different colorants, such as cyan, magenta, yellow, and black (CMYK) inks or dyes, to reproduce the color of a pixel in the original image.
The human eye integrates the individual pixels to form an overall perception of the original image. The accuracy of the reproduction of the original image is a function of the color response of the imaging system. The term xe2x80x9ccolor responsexe2x80x9d refers to a mathematical correlation between the input color values used to control the imaging system and the output color values obtained by measuring the actual colors formed by the imaging system on the substrate. The color response ordinarily must be calibrated to conform to a reference color response. The reference color response may be based on a target imaging system. For example, the imaging system to be calibrated may be a proofing system designed to provide an approximation of an image to be formed on a target printing system.
The calibration procedure typically, involves the application of various combinations of the different colorants to the imaging substrate to form a large number of different color patches. The color patches represent colors distributed throughout the color space of the imaging system. The color patches are measured with a spectrophotometer, colorimeter, or other measuring device, to obtain color values representing the color response of the imaging system. The measured color values may take the form of component color values, e.g., C, M, Y, K, for each colorant present in the measured color patch. Alternatively, the measured color values may take the form of composite color space vectors representing the color space coordinates of the measured color patch in a system-independent coordinate system such as CIE XYZ or L*, a*, b*. The color patches are produced on a target imaging system. The calibration procedure involves mapping the colorant values of the proofing system to produce the same measured color values for the color patches as produced by the target imaging system.
The measured color values are mapped to reference color values representing the color response of the target imaging system after calibration is accomplished. Over time, the imaging system can become uncalibrated due to a variety of hardware and material variations. The variations cause the color response of the imaging system to vary from the color response of the target imaging system. If the deviation is not eliminated, the imaging system will not accurately represent the output of the target imaging system. To eliminate the deviation, thereby recalibrating the imaging system, the input color values must be manipulated.
One method of manipulation is one-dimensional linearization. The term xe2x80x9clinearizationxe2x80x9d refers to one-dimensional correction of the input color values to return the one-dimensional range of the imaging system to its calibrated functional dependence on the one-dimensional domain. This one-dimensional correction ordinarily is accomplished by the application of one-dimensional look-up tables, or xe2x80x9cID LUT""s,xe2x80x9d to the input color values for each separated color. Linearization of a film recorder can be an adequate technique for obtaining consistent color response from conventional halftone color proofing systems. Conventional halftone color proofing systems, such as the 3M Matchprint(trademark) color proofing system, available from Minnesota Mining and Manufacturing Company, of St. Paul, Minn., usually are stable over time and, moreover, are negatively acting. A negatively acting proofing system reproduces an image by removing halftone spots from a substrate, typically by laser exposure and chemical removal, leaving the image portions behind. This approach generally provides a repeatable way of making hard copy color proofs, assuming that the halftone media exhibits a linear response and the output of the proofing system is consistent. Therefore, linearization may be sufficient to return such an imaging system to its calibrated condition.
In contrast, most digital color proofing systems are either continuous tone systems, such as the 3M Rainbow(trademark) digital color proofing system, available from Minnesota Mining and Manufacturing Company, of St. Paul, Minn., or halftone systems with variable maximum densities, such as the Kodak Approval(trademark) digital color proofing system, available from Eastman Kodak Corporation, of Rochester, N.Y. Digital proofing systems can be more susceptible to variation than conventional halftone proofing systems. The various colorants may interact by admixture during the imaging process, for example, significantly altering the color response for secondary and tertiary colors. If the digital proofing system is dye-based, slight changes in the spectral properties of the dyes also tend to be more likely, relative to pigment-based systems, resulting in variations in the color response. Further, variations in the colorant reception characteristics of the substrate can alter the color response.
The larger number of hardware and materials variations arising in direct digital color proofing systems, relative to conventional halftone systems, creates a greater possibility of drift and systematic shift that can affect the color response. Unfortunately, such variations may not be well-corrected by simple one-dimensional linearization of the separated color values in each independent color channel. The colors critical to perception of the image by the human eye tend to lie in regions of color space having more than one color channel that is non-zero in value. One-dimensional linearization of the single color channels independently from one another does not adequately address correction of the color response for admixed colors created by interaction of two or more colorants.
The use of color transformations further complicates the recalibration process. Color transformations are becoming standard techniques in the area of color management. For example, a digital color proofing system will produce a different color gamut and color response than a conventional color proofing system. In order to obtain a good visual match between the output of the digital color proofing system and the output of the conventional color proofing system when proofing the same image file, a complex color transformation typically is required. For CMYK systems, for example, the color transformation is of the form CMYK- greater than Cxe2x80x2Mxe2x80x2Yxe2x80x2Kxe2x80x2. The color transformation is performed on a pixel-by-pixel basis via a LUT or an algorithm. Examples of color transformation techniques are disclosed in U.S. Pat. No. 5,339,176 to Smilansky et al. and U.S. Pat. No. 4,500,919 to Schreiber.
Color transformations can be used to achieve reliable color reproduction. As discussed above, however, the use of color transformations complicates the problem of recalibration of the digital color proofing system. Specifically, the primary colorants C, M, Y, K may each have significant admixtures of other colorants in order to achieve the correct hue. In addition, saturated reds, greens, and blues may have much less than the maximum amount of C, M, Y, or K in order to achieve good color. With color transformation, the individual C, M, Y, and K colorants can no longer be sampled independently of one another. Thus, single-channel linearization is made difficult. Further, standard linearization does not consider second-order effects, such as spatial shifts and interaction between channels.
An alternative recalibration approach considers neutral grays generated after CMYK transformation. This method, implemented by both the above-mentioned 3M Rainbow(trademark) digital color proofing system and the Tektronix Phaser(trademark) 480 color printer, available from Tektronix Incorporated, of Beaverton, Oreg., relies on visual comparison of neutral gray hard copy reference samples and off-gray color patches formed by the system. The off-gray color patches are labeled according to their deviation from gray, i.e., +2% cyan, xe2x88x921% yellow, etc. By visually comparing the reference gray samples to the various off-gray patches, one can determine whether better gray balance is achieved by adding more or less of particular colorants. In order to have an adequate sampling of the different permutations, however, a large number of off-gray patches are required for each gray level, i.e., highlight, quarter tone, midtone, etc.
The visual comparison technique described above works well as a visually based recalibration tool, but is impractical for use as a measurement-based tool. The large number of patches that would have to be measured for a software-based application to determine the optimal corrections required to gray balance the highlight, quarter-tone, mid-tone, and three-quarter tone grays would be extremely time consuming. If saturated chromatic colors, such as reds, greens, and blues were included for reliable accuracy, and multiple samples were measured across the substrate to account for systematic variability, the number of measurements could easily enter into the thousands. Reliable accuracy generally means that any CMYK tint produced by the system will produce substantially the same L*a*b* value as the corresponding CMYK tint produced by the printing or proofing process being simulated, within the noise limitations of both systems.
If the color proofing system without color transformation behaves like a nearly perfect halftone system in its native mode, and if the overlap behavior of the halftone screening process can be approximated as being stochastic, the mathematical calculations to correct for gray balance are fairly simple via the Neugebauer equations. Digital color proofing systems typically are not perfect halftone systems, however, and therefore are less amenable to the use of a measurement-based recalibration tool that directly optimizes the grays and other mixed colors. Moreover, the digital color proofing system may exhibit significant differences in gamut relative to the target proofing system, and therefore requires color transformations, as described above. It is difficult to mathematically calculate the effect on grays and chromatic colors due to alteration of the relative CMYK values, particularly when color transformations are being employed. Hence, visually comparing the large number of three-color gray patches or measuring C, M, Y, and K patches separately are the two primary methods currently employed for recalibration.
The present invention, as broadly embodied and claimed herein, is directed to an apparatus and method for recalibrating a multi-color imaging system.
The multi-color imaging system, in accordance with the present invention, is capable of applying a plurality of different colorants to a substrate based on a plurality of input color values, wherein the input color values control amounts of the colorants, to be applied to the substrate by the imaging system.
In accordance with the present invention, the apparatus and method (a) select a subset of the plurality of input color values, (b) control the imaging system to apply one or more of the different colorants to the substrate based on the subset of the plurality of input color values, thereby forming a plurality of different color patches on the substrate, wherein the subset of the plurality of input color values is selected in the step (a) such that one or more of the different color patches is formed by application of a combination of at least two of the different colorants to the substrate (c) measure a plurality of color values for each of the different color patches formed on the substrate, (d) compare each of the measured color values to a corresponding one of a plurality of reference color values, the reference color values representing a calibrated condition of the imaging system, (e) calculate an error value representing a deviation of the measured color values from the reference color values, and (f) adjust one or more of the plurality of input color values to reduce the error value to a predetermined degree, wherein the adjustment of the input color values for one of the colorants is performed independently of the adjustment of the input color values for others of the colorants.
The advantages of the apparatus and method of the present invention will be set forth in part in the description that follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The advantages of the present invention will be realized and attained by means particularly pointed out in the written description and claims hereof, as well as in the appended drawings. It is to be understood, however, that both the foregoing general description and the following detailed description are exemplary and explanatory only, and not restrictive of the present invention, as claimed.